1. Technical Field
This invention relates generally to gas turbine engines and particularly to the minimization of the fouling of, and minimization of fluid losses within the cooling air passages of turbine airfoils.
2. Background Art
The operation of gas turbine engines is well known. Such engines include a serial arrangement of a fan, a compressor, a combustor and a turbine. Air admitted into the inlet of the engine is compressed by the engine's compressor. The compressed air is then mixed with fuel in the engine's combustor and burned. The high-energy products of combustion of the burned air-fuel mixture then enter the turbine which extracts energy therefrom, in order to drive the compressor and. fan. That energy extracted by the turbine above and beyond that which is necessary to drive the compressor and fan exits the engine at the core engine exhaust nozzle thereof, producing thrust which may power an associated aircraft or operate a free turbine which drives an electrical generator, pump or the like.
A modern gas turbine engine operates at temperatures approaching 3000° F. Accordingly, it is a common practice to cool various components employed in such engines with air provided by the engine's compressor. Perhaps the most critical components to cool with compressor air are the first, and sometimes second stage turbine blades and vanes which are exposed to products of combustion at the exit of the engine's combustor.
It is well known to provide such compressor discharge cooling air to first stage turbine blades and vanes by routing such air through passages internally of a root and subsequently, an airfoil portion thereof. Such passages may be cast into the airfoil portions or, in the case of many large, industrial gas turbine engines, drilled into the blades or vanes by mechanical or electrochemical machining processes. In large industrial gas turbine engines, such cooling air passages often take the form of a series of generally radial holes extending through the entire blade or vane from the root to the tip thereof. Due to the complex shape of such airfoils resulting from curvature and camber of the airfoil portions thereof, as well as the relatively complex dovetail shape of the root portions thereof, it is often impossible to form the cooling air passages by a single drilling operation from root to tip. In most cases, two drilling operations are required for each passage. A first drilling operation forms a first portion of the passages (hereinafter referred to as the root passage) between the root end and a location often in the vicinity of the platform or inner shroud. A second drilling operation forms a second portion of the passage (referred to hereinafter as the airfoil passage) between the tip of the airfoil and the inner end of the root passage.
Since the root portion of the blade is not directly exposed to the hot engine combustion gases as is the airfoil portion, the heat load on the root portion is much less than that of the airfoil portion and thus, the cooling airflow velocities in the root need not be as great as those in the airfoil portion. This heat load characteristic of the blade, coupled with the requirement that the root cooling air passages supply a relatively large volumetric flow of cooling air to the airfoil passages, results in the root passages in most cases being considerably larger in flow area than the airfoil passages. In prior art blades, this difference in flow areas between the root and airfoil passages results in a relatively sharp inside passage corner at the periphery of the inner end of the root passage and a relatively sharp outside corner at the juncture of the airfoil and root passages. It has been observed that contaminants such as fine particulate matter present in the compressor discharge cooling air or combustion gases which may leak into the cooling air passages and contaminants dislodged from the passage walls during the cleaning thereof in the normal periodic maintenance of the gas turbine engine, will, over time, build up in these corner regions and eventually result in partial and sometimes complete blockage of the cooling air passage, resulting in overheating and possible burnout of the blade.